1. Field of the Invention
The present invention relates to a method for producing 4-hydroxybutyl (meth)acrylate which is a monomer suitable for producing paint vehicles used to improve the adhesion property and weather resistance of paints by polymerization thereof.
2. Description of the Prior Art
Generally, when acrylic acid or methacrylic acid is reacted with a diol represented by the formula (1) in the presence of an acidic catalyst, hydroxyalkyl mono(meth)acrylate represented by the formula (2) is obtained, EQU HO(CH.sub.2).sub.n OH (n&gt;2) (1) EQU CH.sub.2 .dbd.CRCOO(CH.sub.2).sub.n OH (n&gt;2) (2)
wherein R represents a hydrogen atom or a methyl group.
In this synthetic reaction, however, alkylene glycol di(meth)acrylate represented by the formula (3) is produced as a by-product at the same time with the desired hydroxyalkyl mono(meth)acrylate, EQU CH.sub.2 .dbd.CRCOO(CH.sub.2).sub.n OCOCR.dbd.CH.sub.2 (n&gt;2)(3)
In this case, formation of the di(meth)acrylate, a by-product, can fairly be inhibited if the molar ratio of a diol to (meth)acrylic acid at the time of feeding is made very large. However, complete inhibition of formation of the di(meth)acrylate, a by-product, is impossible even by such a means. This formation is due to that the reactivity of hydroxyl groups of the diol is substantially equivalent to that of the hydroxyl group of the mono(meth)acrylate, so that formation of the di(meth)acrylate as a by-product is difficult to avoid. Further, there are defects that when the molar ratio of a diol to acrylic or methacrylic acid at the time of feeding is made large, the diol present in a large amount in the reaction system after the completion of the reaction must be separated from hydroxyalkyl mono(meth)acrylate in good efficiency, and besides that the productivity is low because of the diol being used in large amounts.
Also, 4-hydroxybutyl (meth)acrylate and 1,4-butanediol di(meth)acrylate are compounds having boiling points of 230.degree. C. and 275.degree. C., respectively, and being very easily polymerizable and rich in reactivity, so that separation of the both by distillation is almost impossible.
In order to solve these problems, a method is disclosed in which in the continuous production of the monohydroxy ester of a diol by the reaction of the diol with an organic acid in the presence of an acid catalyst, the diester produced as a by-product is separated from the reaction product and returned to the reaction system (DE 1518572 A1). This method converts the diester, a by-product, into the monoester by solvolysis, so that a rise in the yield based on the organic acid can be expected. However the yield based on the diol is so low that an improvement is necessary to use the above method in industry.